Obviously a harmonic vibration. Great idea to check the simple stuff first, like exhaust, exhaust hangars, heat shield, and such. If anything else was touched at the same time, like installing a new trans mount, investigate that as well.
Check the door, dash, trunk, shifter and/or shift boot, center console fit. Maybe when it was installed it stretched another part of the chassis and something is now slightly out of alignment?
Hope you can chase it down easily. Good Luck!
1981 Alfa Romeo Spider 2000 USA
1968 Ford Fairlane
2009 Audi A4 Avant